The structure of our material world is characterized by a large hierarchy of length scales that determines material properties and functions. Increasing spatial resolution in optical imaging and spectroscopy has been a long standing desire, to provide access, in particular, to mesoscopic phenomena associated with phase separation, order, and intrinsic and extrinsic structural inhomogeneities. A general concept for the combination of optical spectroscopy with scanning probe microscopy emerged recently, extending the spatial resolution of optical imaging far beyond the diffraction limit. The optical antenna properties of a scanning probe tip and the local near-field coupling between its apex and a sample provide few-nanometer optical spatial resolution. With imaging mechanisms largely independent of wavelength, this concept is compatible with essentially any form of optical spectroscopy, including nonlinear and ultrafast techniques, over a wide frequency range from the terahertz to the extreme ultraviolet. The past 10 years have seen a rapid development of this nano-optical imaging technique, known as tip-enhanced or scattering-scanning near-field optical microscopy (s-SNOM). Its applicability has been demonstrated for the nano-scale investigation of a wide range of materials including biomolecular, polymer, plasmonic, semiconductor, and dielectric systems. We provide a general review of the development, fundamental imaging mechanisms, and different implementations of s-SNOM, and discuss its potential for providing nanoscale spectroscopic including femtosecond spatio-temporal information. We discuss possible near-field spectroscopic implementations, with contrast based on the metallic infrared Drude response, nano-scale impedance, infrared and Raman vibrational spectroscopy, phonon Raman nano-crystallography, and nonlinear optics to identify nanoscale phase separation (PS), strain, and ferroic order. With regard to applications, we focus on correlated and low-dimensional materials as examples that benefit, in particular, from the unique applicability of s-SNOM under variable and cryogenic temperatures, nearly arbitrary atmospheric conditions, controlled sample strain, and large electric and magnetic fields and currents. For example, in transition metal oxides, topological insulators, and graphene, unusual electronic, optical, magnetic, or mechanical properties emerge, such as colossal magneto-resistance (CMR), metal–insulator transitions (MITs), high-T C superconductivity, multiferroicity, and plasmon and phonon polaritons, with associated rich phase diagrams that are typically very sensitive to the above conditions. The interaction of charge, spin, orbital, and lattice degrees of freedom in correlated electron materials leads to frustration and degenerate ground states, with spatial PS over many orders of length scale. We discuss how the optical near-field response in s-SNOM allows for the systematic real space probing of multiple order parameters simultaneously under a wide range of internal and external stimuli (strain, magnetic field, photo-doping, etc.) by coupling directly to electronic, spin, phonon, optical, and polariton resonances in materials. In conclusion, we provide a perspective on the future extension of s-SNOM for multi-modal imaging with simultaneous nanometer spatial and femtosecond temporal resolution.

中文翻译：

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复杂固体中有序、相和域的纳米光学成像和光谱学

我们物质世界的结构以决定物质特性和功能的大量长度尺度为特征。长期以来，人们一直希望提高光学成像和光谱学的空间分辨率，以提供对与相分离、有序以及内在和外在结构不均匀性相关的细观现象的访问。最近出现了光谱与扫描探针显微镜相结合的一般概念，将光学成像的空间分辨率扩展到远远超出衍射极限。扫描探针尖端的光学天线特性及其尖端与样品之间的局部近场耦合提供了几纳米的光学空间分辨率。由于成像机制很大程度上独立于波长，这个概念基本上与任何形式的光谱学兼容，包括非线性和超快技术，在从太赫兹到极紫外的广泛频率范围内。过去 10 年见证了这种纳米光学成像技术的快速发展，即尖端增强或散射扫描近场光学显微镜 (s-SNOM)。其适用性已被证明可用于广泛材料的纳米级研究，包括生物分子、聚合物、等离子体、半导体和介电系统。我们概述了 s-SNOM 的发展、基本成像机制和不同实现，并讨论了其提供包括飞秒时空信息在内的纳米级光谱的潜力。我们讨论了可能的近场光谱实现，基于金属红外德鲁德响应、纳米级阻抗、红外和拉曼振动光谱、声子拉曼纳米晶体学和非线性光学进行对比，以识别纳米级相分离 (PS)、应变和铁序。在应用方面，我们专注于相关和低维材料作为示例，这些材料特别受益于 s-SNOM 在可变和低温、几乎任意大气条件、受控样品应变和大电磁场下的独特适用性场和电流。例如，在过渡金属氧化物、拓扑绝缘体和石墨烯中，出现了不寻常的电子、光学、磁性或机械性能，如巨磁电阻 (CMR)、金属-绝缘体跃迁 (MITs)、高温超导、多铁性，和等离子体激元和声子极化激元，以及相关的丰富相图，这些相图通常对上述条件非常敏感。相关电子材料中电荷、自旋、轨道和晶格自由度的相互作用导致挫折和退化的基态，空间 PS 超过许多长度尺度。我们讨论了 s-SNOM 中的光学近场响应如何通过直接耦合允许在广泛的内部和外部刺激（应变、磁场、光掺杂等）下同时对多阶参数进行系统的实空间探测材料中的电子、自旋、声子、光学和极化子共振。总之，我们提供了一个关于 s-SNOM 未来扩展的观点，用于同时具有纳米空间和飞秒时间分辨率的多模态成像。